Power supplies are typically used in powering gas discharge lighting, to convert a low-frequency, low impedance, low voltage power source, such as a 120 Volt 60 Hz AC wall outlet, into a high frequency, high voltage and high impedance source suitable for connection to the gas discharge tube light. In typical North American applications, a rectifier rectifies the 120 Volt AC source, and the rectified voltage is filtered to produce an approximately 170 Volt DC source, an oscillator converts the DC source into a high frequency AC source, and a transformer steps up the voltage of this high frequency AC source.
A neon sign (hereinafter also called "neon tubing"), is one example of a gas discharge lamp. Neon signs typically use a transformer (hereinafter also called a "neon transformer") to illuminate the sign. The following discussion of the background and the invention will refer to power supply circuits used for neon signs, however, it will be understood that principles of the present invention have application to power supply circuits for other gas discharge tube lamps as well.
Because the output impedance of a neon power supply is typically relatively large, the output voltage of the neon power supply varies widely depending on the load. Many known neon power supplies suffer from excessive output voltage when operated without load or with a predominantly capacitive load. This may be due to the resonance between the effective transformer output impedance and the output (or stray) capacitance within the transformer or connected to its secondary. Excessive output voltage produces excessive stress on the internal insulation of the transformer, the insulation of the high voltage wiring leading from the neon power supply, and the insulating materials at the supports for holding the gas discharge tube light. The excessive output voltage also violates certain agency safety requirements.
One circuit for preventing excessive output voltages is described by Hopkins et al., U.S. Pat. No. 5,550,437, which is assigned to the same assignee as the present application, and hereby incorporated by reference herein in its entirety. Hopkins et al. describes an neon power supply circuit in which the transformer core incorporates, in addition to primary and secondary windings, a clamp winding. The clamp winding is magnetically closely coupled to the secondary winding via the transformer core, and has terminals connected through a rectifier network to the 170 Volt DC power source produced from the AC line voltage. In an output overvoltage condition, the rectifier network permits current flow in the clamp winding, generating magnetic flux in the core tending to oppose the magnetic flux in the secondary leg of the core, thereby limiting the voltage that can be produced between the secondary terminals. If the overvoltage condition persists, the Hopkins et al. circuit disables the oscillator.
Another concern with known neon power supplies, is the potential that a ground fault from the high voltage outputs of the power supply can create substantial current flows, potentially causing fires if the ground fault creates an arc involving flammable materials. A potentially dangerous ground fault current may occur anytime there is a relatively low impedance path from one of the high voltage output leads of the neon power supply to ground. Such a path may be formed if a neon sign is carelessly installed so that one of the output leads connected to the sign is in contact with a low impedance in a window frame, doorway, or other ground-connected relatively low impedance.
To detect ground fault current, it is typically necessary to couple a ground fault detection circuit to the secondary winding of the power supply transformer, and/or to the neon sign itself. Specifically, the ground fault detection circuit may be coupled between a path to ground, and either a center tap of the secondary winding of the transformer, and/or a return point located near the electrical mid-point of the neon tubing.
Because the ground fault detection circuit is directly connected between ground and the secondary, it must be isolated from the primary side of the main transformer. Accordingly, it is typically necessary to include an auxiliary transformer in the ground fault circuit, to deliver isolated power from the AC source to the ground fault circuit, and/or to transmit an isolated ground fault detection signal to the primary side of the main transformer for the purpose of removing primary power. Unfortunately, particularly where the auxiliary transformer operates at the 60 Hz line frequency, the auxiliary transformer can become prohibitively large and expensive.
A ground fault detection circuit, which does not require an auxiliary transformer, is shown by Pacholok, U.S. Pat. No. 5,089,752. In this patent, the transformer core is used as an isolated, single-wire, "capacitive center tap" to the secondary winding of the main transformer. The theory behind this circuit, is that a ground fault current flowing, for example, into the center tap of the secondary, and out through one of the windings, will create an imbalance between the currents and voltages in the secondary windings on either side of the center tap, which will be manifested as an AC signal at the "capacitive center tap". An AC signal at the "capacitive center tap" thus indicates a ground fault, and can be detected by circuitry on the primary side of the main transformer.
Unfortunately, the Pacholok circuit requires careful balancing of very small parasitic capacitances between the secondary winding and transformer core. Due to variations in these capacitances, and their relatively small values, the Pacholok circuit can be too susceptible to noise and manufacturing variation to detect low-level ground fault currents.
Another ground fault detection circuit which does not require the inclusion of an auxiliary transformer is disclosed in the above-referenced U.S. patent application Ser. No 08/838,060. In the circuit disclosed in therein, electrical energy is collected from current flowing between two secondary windings, and used to supply operating power to a detection circuit on the secondary side of the transformer. The detection circuit detects ground fault currents flowing through an electrical impedance, and generates a shut off signal, which is passed to the primary side of the transformer through an optoisolator, and causes the oscillator to shut down. This circuit, however, requires circuitry on the secondary side of the transformer for collecting electrical energy from the secondary winding current as well as detecting ground fault current flow through the center tap, all of which can add to manufacturing expense.